It is very expensive to drill bore holes in the earth such as those made in connection with oil and gas wells. Oil and gas bearing formations are typically located thousands of feet below the surface of the earth. Accordingly, thousands of feet of rock must be penetrated in order to reach the producing formations. Additionally, many wells are drilled directionally, wherein the target formations may be located thousands of feet laterally away from the well's surface location. Thus, in directional drilling, not only must the depth be penetrated but the lateral distance of rock must also be penetrated.
The cost of drilling a well is primarily time dependent. Accordingly, the faster the desired penetration location, both in terms of depth and lateral location, is achieved, the lower the cost in completing the well. While many operations are required to drill and complete a well, perhaps the most important is the actual drilling of the bore hole. Drilling directionally to a target formation located a great distance from the surface location of the bore hole is inherently more time consuming than drilling vertically to a target formation directly below the surface location of the bore hole.
There are a number of directional drilling techniques known in the art for drilling a bore hole along a selected trajectory to a target formation from a surface location. A widely used directional drilling technique includes using an hydraulically powered drilling motor in a drill string to turn a drill bit. The hydraulic power to operate the motor is supplied by flow of drilling fluid through the drill string from the earth's surface. The motor housing includes a slight bend, typically ½ to 3 degrees along its axis in order to change the trajectory of the bore hole. One such motor is known as a “steerable motor.” A steerable motor can control the trajectory of a bore hole by drilling in one of two modes.
The first mode, called rotary drilling mode, is used to maintain the trajectory of the bore hole at the current azimuth and inclination. In rotary drilling mode, the drill string is rotated from the earth's surface, such that the steerable motor rotates with the drill string.
The other mode is used to adjust the trajectory and is called “sliding drilling” or “slide drilling.” During sliding drilling, the drill string is not rotated. The direction of drilling (or the change in the bore hole trajectory) is determined by the tool face angle of the drilling motor. Tool face angle is determined by the direction to which the bend in the motor housing is oriented. The tool face can be adjusted from the earth's surface by turning the drill string and obtaining information on the tool face orientation by measurements made in the bore hole by a steering tool or similar directional measuring instrument. Tool face angle information is typically conveyed from the directional measuring instrument to the earth's surface using relatively low bandwidth drilling mud pressure modulation (“mud pulse”) signaling. The driller (drilling rig operator) attempts to maintain the proper tool face angle by applying torque or drill string angle corrections to the drill string from the earth's surface using a rotary table or top drive on the drilling rig.
Several difficulties in directional drilling are caused by the fact that a substantial length of the drill string is in frictional contact with and is supported by the bore hole. Because the drill string is not rotating in sliding drilling mode, it is difficult to overcome the friction. The difficulty in overcoming the friction makes it difficult for the driller to apply sufficient weight (axial force) to the bit to achieve an optimal rate of penetration. The drill string also typically exhibits stick/slip motion such that when a sufficient amount of weight is applied to overcome the friction, the weight on bit tends to overshoot the optimum magnitude, and in some cases the applied weight to the bit may be such that the torque capacity of the drilling motor is exceeded. Exceeding the torque capacity of the drilling motor may cause the motor to stall. Motor stalling is undesirable because the drilling motor cannot drill when stalled, and stilling lessens the life of the drilling motor.
Additionally, the reactive torque that would be transmitted from the bit to the surface through the drill string, if the hole were vertical, is absorbed by the friction between the drill string and the borehole. Thus, during drilling, there is substantially no reactive torque experienced at the surface. Moreover, when the driller applies drill string angle corrections at the surface in an attempt to correct the tool face angle, a substantial amount of the angular change is absorbed by friction without changing the tool face angle. Even more difficult is when the torque applied from the surface overcomes the friction in stick/Slip fashion. When enough angular correction is applied to overcome the friction, the tool face angle may overshoot its target, thereby requiring the driller to apply a reverse angular correction. These difficulties make course correction by sliding drilling time consuming and expensive as a consequence.
It is known in the art that the frictional engagement between the drill string and the borehole can be reduced by rotating the drill string back and forth (“rocking”) between a first angle and a second angle measured at the earth's surface. See, for example, U.S. Pat. No. 6,503,48 issued to Richarson. By rocking the string, the stick/slip friction is reduced, thereby making it easier for the driller to control the weight on bit and make appropriate tool face angle corrections. A limitation to using surface angle alone as a basis for rocking the drill string is that it does not account for the friction between the wall of the bore hole and the drill string. Rocking to a selected angle may either not reduce the friction sufficiently to be useful, or may exceed the friction torque of the drill string in the bore hole, thus unintentionally changing the tool face angle of the drilling motor. Further, rocking to angle alone may result in motor stalling if too much weight is suddenly transferred to the bit as friction is overcome.
Another difficulty in directional drilling is controlling the orientation of the drilling motor during sliding drilling. Tool face angle information is measured downhole by a steering tool and displayed to the directional driller. The driller attempts to maintain the proper face angle by manually applying torque corrections to the drill string. However, the driller typically over- or under-corrects. The over- or under-correction results in substantial back and forth wandering of the tool face angle, which increases the distance that must be drilled in order to reach the target formation. Back and forth wandering also increases the risk of stuck pipe and makes the running and setting of casing more difficult.
A further difficulty in directional drilling is in the transitions back and forth between sliding drilling and rotary drilling. Substantial reactive torque is stored in the drill string during both sliding and rotating drilling in the form of “wraps” or twists of pipe. During drilling, the drill string may be twisted several revolutions between the surface and the drilling motor. Currently, in transitioning between sliding drilling and rotary drilling (and vice versa), the bit is lifted off the bottom, which releases torque stored in the drill string. When drilling resumes, the bit is lowered to the bottom and the reactive torque of the steerable motor must be put back into the drill string before bit rotation resumes to a degree such that earth penetration is effective. Moreover, when sliding, drilling commences, the driller has little control over the tool face angle until the torque applied to the drill string stabilizes at about the amount of reactive torque in the drill string, which adds to the difficulties inherent in controlling direction. As a result, slide drilling has proven to be inefficient and time consuming.